Pump for Two Liquid Phases 1

[sshep]

My Friends,

We have a pump that struggles on overcurrent that pumps a two phase mix of hydrocarbons and water off a tower bottoms. This pump was designed on the edge because it is not rated to pump just water, but rather an assumed SG for the mix at SG=0.85. The pump design efficiency is 18% (not a typo). Out of ideas, we sent this pump to be water tested at an independedt shop and it is on its design curve.

The SG used for design was weighted average of the SG of hydrocarbon and water (design 7 wt% water). Because it is two phase, it strikes me that there could be centrifugal effects, and that an "weighted average" SG is not the right approach.

Do you guys have any insite on this subject?

Please know that I didn't design this crap system, I'm just stuck with it.

best wishes,
sshep

 

[StoneCold]

What about putting an orfice in one of the flanges down stream to bring the pressure drop up and push the pump back a little on it's curve. If it is intermittant pumping this could help you quite a bit. Is it a full speed pump (3600 rpm) or half speed.
Seems like when the pump is not working well they also went cheap and it is a full speed pump.

Regards
Stonecold

 

[DubMac]

sshep,
When you say "two-phase mix of hydrocarbons", you don't mean there is a gas phase present do you?? If so, then you have entered a whole new world of pumping... I am assuming you just mean you have water and oil mixed together, both in the liquid phase.

What type of pump do you have? Centrifugal? Positive displacement? 18% is really low for any pump; sounds like you are operating WAY off of the original design point of the pump, and easiest fix may be to find a way to "move" the operating point back to higher efficiency. StoneCold suggests one way to "move" the point to the left, but would really help to see a copy of your performance curve if possible.

 

[Latexman]

Overcurrent? Add a restriction in the discharge, add a VFD, or put in a bigger motor.

Good luck,
Latexman

 

[DubMac]

good job Latexman....quick and to the point.

 

[sshep]

Hey My Friends,

I attached a picture of the impeller for this centrifugal pump which is unusual and probably explains the low efficiency. This is made by FindeR Pompe, an Italian company, model 1x2-PEP-131-BL, the only one like it in the plant. Perhaps it was the cheapest that they could get made of 904SS because it is handling a mix of water and acids from the bottom of extract tower of a process that does LL extraction of acids.

It was discovered sometime back that the overcurrent trips had been bypassed for years, and the pump is operated with every bit of throttling possible to keep it from tripping- i.e. no min flow, no warm up flow for standby pump, etc.

It occurred to me after this post that the % water in the fluid is not going to be constant, but may rather come in slugs. This may be the real problem with using an average SG assumption on such a tight design. We hope to mount a size larger motor- you know the drill (cabling, motor starter, baseplate, etc), otherwise we buy new. In the end the root cause is a really shabby original design, probably the cheapest installed cost to meet the 904SS material without any concern for the operability.

Have you ever seen an impeller like this?

best wishes,
sshep

 

[Pumpsonly]

This type of impeller design is for low flow high head pump similar to the Sundyne pumps which are very low specific speed. It is very inefficient but you won't find many alternative make that can give you the head and flow for a single stage and direct driven.

Sundyne LMV 322 may be an alternative, but I don't think you get any better efficiency.

You may look at Envirotech also. Don't know who owns it now.

 

[sshep]

Hello Pumpsonly,

I see you are right on. Having checked out what some other pump vendors offer for single stage pumps in similar service (rated: 15.3 cum/h, 175m head), the efficiencies are all dismal and generally less than 25%. This bites! Oh well, I will see what I can do about reducing the head and hope that we can mount a size larger motor.

Thanks alot,
sshep

 

[Pumpsonly]

When you have to pump varying density pumpage with centrifugal, you size the head using the lowest SG and size the motor using the highest SG.